There is great interest with respect to benefits such as fast reading speed, fast writing speed, superior durability, nonvolatility, and low power consumption of magnetic random access memories (MRAMs). The MRAM is a nonvolatile memory including a giant magnetoresistive (GMR) element or a tunnel magneto resistance (TMR) element as a memory element, which stores information in the memory element.
One example of a MRAM is a spin transfer torque magnetic random access memory (STT-MRAM) in which a magnetization of a magnetic material is switchable by causing a current to flow into the magnetic material. A magnetization in a nanoscale magnetic material is easy to be controlled in a local magnetic field when the spin transfer torque method is used. The current for switching the magnetization is also expected to be low as the magnetic material is scaled down. In STT-MRAMs, switching of the magnetization caused by a read current is controlled by using a large current for writing and a low current for reading, and by using the same terminals.
The thermal stability of a magnetic material is given by an index Δ (=KuV/kBT), where Ku represents the magnetic anisotropy of the magnetic material, V represents the volume of the magnetic material, kB represents the Boltzmann constant, and T represents an absolute temperature. In order to maintain the index Δ while miniaturization of the magnetic material is pursued, it is necessary to increase the magnetic anisotropy Ku. An increase in the magnetic anisotropy Ku requires a larger write current. Therefore, maintaining the magnetic anisotropy Ku is a trade-off between the decreasing write current and miniaturization (high density). Furthermore, writing error rates of the magnetic material are increased by the magnetization switching of the magnetic material caused by the read current.
Consequently, a method for using the spin Hall effect or the spin orbit interaction, in which a write current terminal and a read current terminal are separated from each other to lower the writing error rates, is proposed. This method improves writing error rates. However, it is a known fact that a spin Hall angle ΘSH varies with the film thickness of a layer having spin orbit interaction (also referred to as a SOL (Spin Orbit Layer) hereinafter). The spin Hall angle represents the ratio of spin transmission to electric conductivity. When the current density in the SOL is constant, the value of the current increases with the thickness of the SOL. Therefore, it is important to suppress the thickness of the SOL.
However, when the SOL is thin, damages occur during manufacturing of magnetoresistive elements, wires break due to excessive etching and electromigration is caused when applying current. When damages or excessive etching are suppressed, the etching of portions of the magnetoresistive element is insufficient and the side walls of the magnetoresistive element have a tapered shape. As a result, the elements increase in size or there is an increase in size variation. Thus, there is a trade-off between a low write current due to a thinner SOL and low characteristics of the SOL.